Process for storing a gas in a coal mine



3,462,957 PROCESS FOR STORING A GAS IN A COAL MINE Yvon Henri ArthurLoir, 97 Route de Mons, Fontaine-LEveque, Belgium No Drawing. Filed May1, 1967, Ser. No. 634,824 Claims priority, applicatiorli Belgium, May 5,1966, 27 7 Int. Cl. E21f 1i/16; B65g 5/00 US. Cl. 61-.5 1 Claim ABSTRACTOF THE DISCLOSURE The present invention relates to a process for storageof gas, especially combustible gas, in an underground reservoir or gasholder.

Underground storage of combustible gas has already been effected incertain geological formations.

According to a known process for storing natural gas, the latter isinjected into a spent petroleum or natural gas bed or stratum. Thestorage capacity of the under ground reservoir formed by this bed is,among other factors, a function of the free space remaining in the saidbed due to its previous working.

According to another known process for storing natural .gas, the latteris injected into a previously worked, abandoned coal mine filled withground water. Such a mine forms a great U-shaped underground reservoirwith at least two shafts which serve respectively to convey water andgas. In this reservoir, gas is above the adjustable level of water,under a pressure corresponding to the height difference between saidlevel and the surface of the ground. The storage capacity of saidreservoir is proportional to said pressure and depends on the gas volumeincreasing when exhausting water from the mine. In such an undergroundreservoir which works according to the principle of communicatingconduits, the presence of water reduces the possibility of diffusion,fixation, absorption and adsorption of the gas in the rock of the mine,namely through decreasing of the rock permeability. The storage capacityof said reservoir is thus practically limited to the geometrical volumeof the mine cavities above the water level in said mine.

According to another known process for storing combustible gas, thelatter is introduced into porous sandbearing rocks, which are saturatedWith water and confined under an impermeable perianticlinal dome. Inthis case, in order to prevent an inrush of water into the shafts ofboreholes, the injection of a Ininirnum permanent volume of gas isindispensable when putting the reservoir into service (a cushion orgas-lock). The storage capacity of the underground reservoir formed bythese rocks is then inferior to the actual inherent volume of thisreservoir, Since the introduction of the gas into these rocks involvesthe pressing back of the water in these and since the extraction of thisgas out of the said rocks is effected under the counter-pressure of thiswater, these reservoirs have to have a very large gas-water contactsurface and must then be enormous having regard to their effectivecapacity. In addition, the gas injected is not necessarily in chemicalequilibrium with the rock of the reservoir and there is thus a risk ofpollution of the waterbearing stratum.

'nited States Paten 0 ice According to other known processes for storingcombustible gas, the gas is introduced into natural or artificialunderground reservoirs found in suitable impervious geologicalstructures such as salt formations, non-fractured coherent rocks(argillaceous limestones, schists, crystalline rocks, quartzites,limestones) or plastic rocks (clays). In these latter processes, thefilling of natural or artificial geometrical cavities of the stratum iseffected. The true storage capacity of these reservoirs is directlydependent on the available geometrical volume of the cavities.

The object of the present invention is a new process for storing gas,especially combustible gas, in which the true storage capacity of theunderground reservoir is greater than its own inherent volume.

To this end, according to this new process, the gas to be stored isintroduced into the gassy coal deposit previously partially worked,degassed and unsaturated with water, at a relatively high pressure so asto increase the absorption of said gas in the rocks of the coal depositand, in particular, the adsorption of said gas by said rocks.

The fact that the true storage capacity of the coal deposit isappreciably greater than the inherent geometrical volume of this measureis explained by the property which rocks of coal deposit have of beingable to absorb a large quantity of gas amongst others under the effectof pressure, and this quantity of gas absorbed may attain, for example,40 m. N of methane per metric ton for some coals.

Thus, when a gas is introduced into a coal deposit, it not only fillsthe cavities, fissures and pores of this deposit but it is also absorbedby the rocks constituting the said deposit.

Therefore, according to the process one can introduce into a coaldeposit a quantity of gas, especially combustible gas, greater then thatwhich could be introduced into other geological formations by the knownprocesses, that is, for the same inherent volume of all these strata andfor the same injection pressure of the gas therein.

The introduction of the gas into the underground reservoir may be madein an unworked coal deposit. However, a coal mine which has previouslybeen partially worked, degassed and unsaturated with water maypreferably be used to store this gas. Such a mine has in fact theadvantage of having an inherent volume which is larger than that of thenot yet worked deposit and which is due especially to the residualvolume of the cavities excavited during he mining operations. Inaddition, such a mine has also an enormous gas-rock contact surfacerepresented by the surface of all these excavated cavities as well as bythe multitude of cracks and fissures caused by the movements of therocks due to these mining operations.

This considerable fracturing of the deposit increases the permeabilityof the rocks unsaturated with water and thus facilitates the circulationand diffusion of the gas in the said mine or deposit.

. On the other hand, the partial removal of gas or the desorption ofthecoal deposit surrounding the part of the inherent volume formed bythe mining operations facilitates the absorption of the gas injectedinto the mine and in particular, the adsorption of this gas by the rocksof the said deposit.

According to the invention, the gas is introduced into the coal depositwhich is previously and partially worked and degassed, also unsaturatedwith water.

Furthermore, the diffusion and absorption of gas in the rocks of a coaldeposit result in phenomena which depend on the internal structurecharacteristic of some rocks of the coal deposit (coals and coal-bearingschists). The specific internal surface of some coals may attain, forexample, mP/gram. The gaseous exchanges between the gas outside the rockand that absorbed by the internal structure of the rock dependespecially on the nature and the pressure of the external gas in contactwith the rock, the magnitude of the contact surface between the externalgas and the rock, the nature, internal structure, permeability,magnitude of the internal surface, temperature, moisture and adsorbentpower of the rock. In the present specification, the external gaseousphase means the gas found in the part of the inherent volume of themeasure which is exterior to the rock and which is formed by theresidual volume of all the cavities due, for example, to former miningoperations and to the shafts for access to the deposit; the internalgaseous phase means the gas found in the part of the inherent volume ofthe deposit which is inside the rock and filling the free spaces of itsinternal structure; and adsorbed phase means the gas found in theinternal structure of the rock and covering the internal surfacethereof.

The internal phase and the adsorbed phase of the gas which are adsorbedby the internal structure of the rock are normally in equilibrium withthe external gaseous phase, this equilibrium depending among otherfactors on the pressure of this external gaseous phase.

In particular, the result is that the diffusion of the gas into therocks of the coal deposit and the absorption of this gas by these rocksare facilitated by an increase of pressure in the external gaseousphase.

On that account, according to the invention, the gas to be stored isintroduced at a relatively high pressure in order to facilitate theabsorption of this gas in the rocks of the coal deposit.

The adaptation of a gassy coal mine, which has been previously andpartly worked, degassed and which is unsaturated with water, to anunderground reservoir for combustible gas is specially advantageousbecause it does not necessitate the injection of any gas for washing andabove all because it utilises the pit gas or methane remaining up tothen absorbed in the coal mine. To this end and according to animportant feature of the invention, there is progressively extractedfrom the gassy coalmine gas from the external phase previously found inthe precinct defining the external volume of the rock of the undergroundreservoir so as to cause it to pass into this external gas from the pitgas up to then absorbed in the rocks of the deposit. In this way, thecalorific power of the gas of the external phase is gradually increasedand it is regulated with relation to that of the gas to be stored inorder to make is approximately equal to the latter in thermal power.Such an extraction of the external gas from the gassy coal-mine iseffected before any injection of gas to be stored into the undergroundreservoir formed by this mine. The preparation of a storage reservoirfrom a gassy coal mine previously partly worked, degassed andunsaturated with water is thus both simple and particularly economical.

Other details and features of the invention will appear during thedescription of an example of storage of combustible gas, given withoutlimiting the invention thereto.

The case envisaged concerns the storage of a natural combustible gashaving, for example, the following mean composition:

Percent CH 81.3 C H 2.9 C H 0.4 04H) 0.1 C H 1.1 co 0.8 N 14.4

formed on one hand by the shafts, galleries, workings intentionallyexcavated during the working and on the other hand, by cracks, fissuresand cavities produced during and after this working.

In the example selected, it may be estimated that this residual volumeof the underground reservoir formed in this coal mine by the miningoperation is about 10,000,- 000 m.

This volume external of the rock of the underground reservoir is filledwith a gas which is denoted by external gas or external gaseous phase.

Before the preparation for the storage of gas in this reservoir, thisexternal gas is from the air of the mines having, for example, thefollowing average composition:

Percent CO 0.5 CH 2 O 18.5 N 79 On the other hand, this external gas isfound at a pressure of the order of 1 atmosphere and has a calorificpower of about calories/m. N.

The above-mentioned external volume of the underground reservoir isdefined by the rocks of the coal deposit, which include among otherscoals and schists.

These rocks contain a certain amount of absorbed pit gas or methane.This absorbed pit gas is composed of an internal gas and an adsorbed gasor, in other words, an internal gaseous phase and an adsorbed phase.This absorbed pit gas is at a pressure of about 1 atmosphere in thevicinity of the contact surface between the external gas and the rocksand which increases progressively, for example up to 50 atmospheres, inproportion as it is removed from the degassed and desorbed zone of thiscontact surface towards the non-desorbed and virgin zone of the coaldeposit.

This pit gas absorbed by the rocks of the coal-measures hasapproximately the following average composition:

Percent H 0.133 CH 97.2 C H 1.96 N +Ar+Kr 0.32 He+Ne 0.0257 CO 0.38 H 8Traces and an average calorific power of about 9600 calories/ in. N.Before any storage operation in such a previously Worked, gassy andunsaturated with water coal deposit, there exists in the interior of thesaid deposit an unbalance of the pressures of the external gaseous phaseand the internal and adsorbed phases. This pressure gradient causes amigration of the pit gas absorbed in the rocks of the coal deposittoward the external gaseous phase.

The underground storage reservoir is prepared from this worked coal mineby a first simple operation consisting in extracting, after sealing theshafts, the gas of the external phase while maintaining it at lowpressure with respect to that of the internal and adsorbed phases.

For example, the external gaseous phase is maintained at a pressure of 1atmosphere by drawing off while the internal and adsorbed phases of thevirgin deposit are at a pressure of about 50 atmospheres.

The maintenance of this unbalance between the pressure of the externalgaseous phase and the internal and adsorbed phases of the rocks of thedeposit facilitates the migration of the pit gas absorbed in these rockstowards the external gaseous phase.

The coal mine is degassed and desorbed from its constituent pit gas bythe extraction in question, while it is enriched in methane, the saidexternal gaseous phase. The calorific power of this external gaseousphase is thus increased and the adjustment of this calorific power canbe carried on by the single extraction mentioned above until, forexample, the following average composition is attained:

corresponding to a mean calorific power of about 8300 calories/m. N,equal to that of the calorific power of the gas to be stored.

When this adjustment is finished, the gas to be stored is introducedinto the underground reservoir-formed by the said coal mine. Thisintroduction of the gas to be stored is made preferably under arelatively high engine pressure but able to be varied, for example, from1 to 50 atmospheres according to the amount of gas to be stored.

Throughout and after the introduction of the gas to be stored in theunderground reservoir formed by the coal mine, the external gaseousphase is also found at this variable pressure of 1 to 50 atmospheres andconsequently this external gaseous phase which fills the volume externalto the rocks of the deposit, partly diffuses into these degassed,desorbed and unsaturated with water rocks Where it is absorbed. Thesequantities of gas absorbed by these rocks increase respectively theinternal and adsorbed phases, the internal pressure of which increasesas a function of the pressure of the external gaseous phase.

Owing to this fact, the storage capacity of the underground reservoirformed by the aforesaid coal mine is by no means limited to that of theinherent volume of the mine and, under a pressure of 10 atmospheres, astorage capacity of, for example, 500,000,000 111. N may be obtained.

Since the phenomena of diffusion and absorption of the gas into therocks of the coal mine are reversible as a function of the equilibriumfactors between the external gaseous phase, the internal gaseous phaseand the adsorbed phase, the operations of removal from storage areeasily carried out by reduction of the pressure of the external gaseousphase.

What I claim is:

1. Process for storing hydrocarbonaceous gas, comprising withdrawingfrom a gassy coal mine which has previously been partially worked andwhich is unsaturated with water, the gas which is external to the rockof the coal deposit of said mine, until the heating value of the gasbeing withdrawn is at least about equal to that of the gas to be stored,and thereafter introducing said gas to be stored into said coal mine atsuperatmospheric pressure so as to promote adsorption of said gas to bestored by the partially worked coal formation.

References Cited UNITED STATES PATENTS 2,508,949 5/1950 Howard 61.5 X2,810,263 10/1957 Raymond 61--.5 2,817,235 12/1957 Hunter et al. 61-.5 X3,152,640 10/1964 Marx 61.5 X

EARL J. WITMER, Primary Examiner

